Several devices have been developed for injecting predetermined quantities of liquid additives into a primary liquid stream for such applications as adding medication to drinking water for livestock, treating water with additives such as halogens, or adding fertilizer concentrate to irrigation water, for example. In known devices, energy supplied to the pumping mechanism originates from the flow of the primary fluid under pressure in an enclosure containing a stepped differential piston. A mechanism with valves carried by the piston enables the fluid pressure to be applied to either face of the stepped piston, which thus describes a reciprocating motion and which forms the driving member for a metering piston interacting with a cylinder in communication with a storage vessel of the product to be injected. Such devices are found in my own U.S. Pat. Nos. 4,558,715 and 4,809,731, as well as U.S. Pat. No. 4,756,329 to Jean Cloup.
It is to be understood that although the additive secondary fluid to be injected into the main stream by an injection pump is usually a relatively small volume compared to the main fluid volume, the additive fluid may not be a true solution, but rather a dispersion solution which contains abrasive particles. This may be a result of chemical precipitation upon contact with the primary fluid because of temperature changes and chemical conditions. Accretion of hard precipitate can occur on surfaces and parts simply by passing ordinary tap water thereover after an extended period of time.
Some of the injection devices, such as my U.S. Pat. No. 4,558,715, may have a seal through which the injection pump piston rod reciprocates in order to direct the additive fluid to the outlet side of a reciprocating motor. Others, such as U.S. Pat. No. 4,756,329, have an injection pump with a metering piston which slides in a tube or liner which is exposed to the secondary additive fluid. The prior art injection pumps are thus subject to wear which destroys their ability to pump and to draw secondary fluid into the piston cylinder chamber. Wear may also destroy seals intended to separate the primary and secondary fluids which may cause the primary fluid to dilute the secondary fluid in the additive pump and result in imprecise metering. It would be desirable to eliminate sliding surfaces which wear and leak.